1. Field of the Invention
The present invention relates generally to skin grafting and related devices and methods. The present invention provides a systematic approach to the process of skin grafting, i.e., harvesting, post-excision processing and application of donor skin and treatment of the graft recipient site.
2. Description of the Related Art
Advances in medical technology have provided many patients with benefits inconceivable a century ago. In particular, skin grafting has enabled doctors to heal wounds with the patient's own skin from a harvest site on the patient. The skin grafting techniques have many wonderful benefits, but are still replete with a number of problems.
The process of split-thickness skin grafting can be envisaged as a series of steps; (1) harvesting the split-thickness-skin graft (“STSG”) at a donor site; (2) processing of excised STSG; (3) application of the processed skin to the wound site; and (4) pre- and/or post-graft treatment to accelerate healing of the wound site. Each of these steps interposes various challenges and obstacles, e.g., technical, therapeutic and financial, in executing a successful graft.
In regard to the first step, harvesting a STSG at a donor site has traditionally been accomplished using powered, hand-held dermatomes. These devices are expensive and the operation is known to be highly dependent on user skill and training, and requires involved procedures to accurately obtain a successful harvest. These devices must be operated at a precise constant angle relative to the skin, with the exact amount of pressure to insure a uniform harvest. Slight variations in operative use of these dermatomes result in excised skin of variable-thickness, which sometimes must be discarded altogether. As a result, these devices are primarily wielded only by experienced plastic surgeons. Use of these dermatomes are generally confided to the operating room setting, increasing the cost of the procedure, especially given the average fee for operating room use.
There is a current need for harvesting procedures that require a lower degree of operator skill and are capable of being performed outside of an operating room, thus decreasing the costs of the procedure.
In regard to the second step of processing excised skin, it is highly desirable to maximize the coverage of the donor skin at the wound site for any given area of a potential donor site. Apart from minimizing trauma incurred at the donor site, a major factor limiting survival following extensive injury is insufficient availability of donor sites to provide enough skin for the required grafting procedures. One procedure is to mesh the skin graft i.e., creating slits in the excised donor skin to allow for the skin to be stretched. A graft-meshing machine is commonly used in hospital-based surgical practices, and generally allow for an expansion ratio of 3:1 to 9:1. The excised harvested skin is placed on a specific template, depending on the expansion ratio desired, and the template and graft are pressed through the mesher. While greater ratios than 9:1 may be possible using meshing techniques, there is a concomitant significant delay in epithelization with using such ratios. When healed, a meshed grafted site characteristically has a permanent “crocodile skin” or “weaved” appearance.
Micro grafting techniques, in which the donor tissue is actually minced in order to achieve a greater than 10:1 expansion ratio, are known in the art. Such techniques allow for a greater coverage area from a small donor site than meshing techniques. Traditional micrograft techniques, dating back to 1963, utilized minced skin that is between ⅛th inch (approximately 3 mm, or 3000 μm) and 1/16th inch (approximately 1.5 nm, or 1500 μm) in size. However, disadvantages of using pieces larger than 1500 μm have been noted. For example, in skin pieces of this size cells remote from a cut edge have a limited availability to migrate and proliferate and thereby contribute to forming new skin. In addition, the techniques employed have required each piece to be oriented epidermis upwards, making the procedure tedious and impractical. Further, the appearance of the new skin that is produced using particles of this size is poor, often having a cobblestone appearance.
There is currently a need for a procedure capable of producing micrograft particles in a size less than 1500 μm in a rapid and efficient manner, with a minimum of handling procedures, while resulting in skin pieces that are viable and capable of “taking” when applied to a wound site. Such technique would significantly aid in the ease and speed of operations utilizing micrografts.
The third step of the graft procedure, application of processed excised skin to the wound site, it is particularly relevant to the application of micrograft particles to a wound site. The effect of a decrease in size on micrograft particles on the inter-particle distance for a given expansion ratio is shown in schematic form in FIG. 27. Given that the epitheliaztion of the inter-particle wound surface is affected by migration and proliferation of keratinocytes from the micrografts, it is readily evident that the most effective distribution is that idealized in FIG. 27. Current methods of distributing micrografts, such as mechanical spreading results in clumps or aggregates of skin particles, frustrating an even distribution. In addition, in larger aggregates, some micrograft particles will not be in direct contact with the wound bed. Such particles cannot readily integrate with the wound bed and also will have a reduced potential for nourishment from the wound fluid exudates and consequently have an decreased potential to remain viable. Thus, the aggregation of micrografts reduces the efficiency of epithelization and may significantly increase the time required to close a wound.
There is a current need for devices and methods to effect an even distribution of micrcograft particles on a wound surface, thereby promoting the efficiency of epithelization.
The fourth step of the graft procedure relates to pre- and/or post-graft treatment to accelerate healing of the wound site. As is known in the art, closure of surface wounds involves the inward migration of epithelial, dermal and subcutaneous tissue adjacent to the wound. This migration is ordinarily assisted through the inflammatory process, whereby blood flow is increased and various functional cell types are activated. Through the inflammatory process, blood flow through damaged or broken vessels is stopped by capillary level occlusion; thereafter, cleanup and rebuilding operations may begin.
When applied over a sufficient area of the wound, negative pressures have been found to promote the migration toward the wound of surrounding cutaneous and subcutaneous tissues. In practice, the application to a wound of negative gauge pressure, commercialized by Assignee or its parent under the designation “Vacuum Assisted Closure” (or “V.A.C.®”) therapy, typically involves the mechanical-like contraction of the wound with simultaneous removal of excess fluid. In this manner, V.A.C.® therapy augments the body's natural inflammatory process while alleviating many of the known intrinsic side effects, such as the production of edema caused by increased blood flow absent the necessary vascular structure for proper venous return.
Despite being highly successful in the promotion of wound closure and healing many wounds previously thought largely untreatable, some challenges remain with the use of negative pressure therapy. One such area is in the frequent changing of the wound dressing. As the wound closes, binding of cellular tissue to the wound dressing may occur and use of traditional V.A.C.® therapy necessitates regular changing of the dressing. If cellular tissue has grown excessively into the dressing, there is the potential that dressing changes without due care and attention may result in some tissue damage at the wound site. The problem of frequent dressing changes would likewise tend to frustrate the combination of micrografts with negative pressure treatment, in that the dressing change may remove or displace micrografts.
There is current need for methodologies to address the challenges involved with frequent dressing changes and to effectively combine the micrograft techniques with negative pressure treatment to effect enhancement of wound closure.